Fuel tanks for storing liquid fuel are used, for example, for transporting or storing fuel and on vehicles for their propulsion systems. The volume of gas above the liquid fuel within the fuel tank is known as the ullage. The ullage can contain reactive components, such as oxygen and/or fuel vapours, which can be a potential fire/explosion hazard. The term “inerting” refers to the process of depleting one or more of these reactive components to reduce this potential hazard.
Various inerting systems are known in the art. For example, the potentially explosive atmosphere in the ullage can be displaced by the addition of inert gas, such as Argon, Nitrogen, or Carbon dioxide. Particularly in the case of vehicle fuel systems, the ullage volume increases as the liquid fuel is consumed and so the inert gas needs to be added while the vehicle is in motion. Typically, the inert gas is either stored in a pressurised tank on the vehicle for use on demand, or generated on board the vehicle for immediate use.
On board generation of inert gas can have several advantages in terms of reduced weight and maintenance requirements, which are important considerations, particularly for the aircraft industry. Some known On Board Inert Gas Generation Systems (OBIGGS) for aircraft pass engine bleed air through a gas separator to produce oxygen depleted air, which is fed to the ullage. More recently, catalytic inerting systems have been proposed (see, for example, WO 2007/008730) which take oxygen and fuel vapour from the ullage and pass these over a hot catalytic bed to oxidize the fuel vapour and produce Carbon dioxide and water vapour. The Carbon dioxide inert gas is fed to the ullage in a closed loop. Where there is a negative pressure differential between the ullage and ambient, there is a net inflow of oxygen rich ambient air into the fuel tank through its vent. This negative pressure differential can arise due to a reduction in fuel level in the tank as the fuel is being consumed, and also due to changes in ambient pressure due, for example, to a decrease in height. Whilst the oxygen in the air which enters the ullage through the vent will, over time, be fed to the catalyst to be consumed in the catalytic reaction, it is desirable to prevent this oxygen rich air from entering the ullage in the first place.